Business, science and entertainment applications depend upon computers to process and record data, often with large volumes of the data being stored or transferred to nonvolatile storage media, such as magnetic discs, magnetic tape cartridges, optical disk cartridges, floppy diskettes, or floptical diskettes. Typically, magnetic tape is the most economical means of storing or archiving the data. Storage technology is continually pushed to increase storage capacity and storage reliability. Improvement in data storage densities in magnetic storage media, for example, has resulted from improved medium materials, improved error correction techniques and decreased areal bit sizes. The data capacity of half-inch magnetic tape, for example, is now measured in hundreds of gigabytes on 512 or more data tracks.
The improvement in magnetic medium data storage capacity arises in large part from improvements in the magnetic head assembly used for reading and writing data on the magnetic storage medium. A major improvement in transducer technology arrived with the magnetoresistive (MR) sensor originally developed by the IBM® Corporation. Later sensors using the GMR effect were developed. AMR and GMR sensors transduce magnetic field changes to resistance changes, which are processed to provide digital signals. Data storage density can be increased because AMR and GMR sensors offer signal levels higher than those available from conventional inductive read heads for a given read sensor width and so enable smaller reader widths and thus more tracks per inch. Moreover, the sensor output signal depends only on the instantaneous magnetic field intensity in the storage medium and is independent of the magnetic field time-rate-of-change arising from relative sensor/medium velocity. In operation the magnetic storage medium, such as tape or a magnetic disk surface, is passed over the magnetic read/write (R/W) head assembly for reading data therefrom and writing data thereto.
The quantity of data stored on a magnetic tape may be increased by increasing the number of data tracks across the tape. More tracks are made possible by reducing feature sizes of the readers and writers, such as by using thin-film fabrication techniques and MR sensors.
The interleaved R/W magnetic tape head with MR sensors allows increased track density on the tape medium while providing bi-directional read-while-write operation of the tape medium to give immediate read back verification of data just written onto the tape medium. Tape recording systems may alternatively implement arrays of “piggyback” R/W pairs, where the writer and reader in each pair are aligned in the direction of tape travel.
In any of the above-mentioned types of heads, e.g., AMR, GMR, MTJ, etc., several forces are present, each potentially contributing to adverse effects on the head. Particularly, the tape acquires a charge as it moves through the tape path and over the head. In the head, the sensor element potential is generally set by virtue of the circuit that provides the sensing current passing therethrough. The sensor shields may or may not have a potential matching the sensor element. Further, the sensor shields and other parts of the head may become charged by interaction with the tape. Thus, as can be appreciated, multiple and varying voltage differentials may be found not only between the tape and the head, but also between various components in the head, e.g., substrate, shields and sensor element. As will soon become apparent, these voltage differentials are believed to be at least partly responsible for adverse tribological interactions on the head.
Compounding the problem is the electric field distribution in the head ceramic. The wafer material used for magnetic recording heads (commonly know as AlTiC), is a ceramic composite material consisting of a matrix of insulative aluminum oxide (alumina, Al2O3) plus an irregular but interconnected network of electrically conductive titanium carbide (TiC). The uneven distribution of TiC creates an irregular electric field at the interface with the typically overlying insulator. The electric fields at grain edges can be several folds higher than the average field. These high fields are associated with adverse tribological effects, including electrostatic debris deposition and pitting of the head insulator due to electric discharges. These high fields also promote electrical discharge into the tape, thereby charging the tape.
Due to the varying voltage differentials and irregular electric fields between the various parts of a head as well as between head and tape, magnetic heads tend to suffer from adverse tribological interactions, which include electrical discharge, tape changes, head erosion, debris buildup, chemical conversion, head sensor shorting, etc. In both piggyback and interleaved heads, tribological effects are believed to be aggravated by excessive substrate and/or media voltages. When the substrate is electrically floating, relative motion between the head and recording medium may produce substrate and media voltage swings on the order of several 10s of volts. Such voltages are strongly implicated in unfavorable tribological processes such as electrochemical reactions, electrostatic accumulation of debris, and even certain types of wear.
In shielded MR heads, the lower reader shield of each reader is in close proximity to the substrate, separated therefrom by a thin insulator on the order of 1 micron thick. The voltage differences between the adjacent reader shield and substrate is problematic due to their close proximity, and is compounded by the potentially large localized electric fields created by a conductive substrate, e.g., of AlTiC. Such electric fields are implicated in aggravated accumulation of conductive materials that can actually short the MR sensor to its shields and in turn to the substrate.
Consider the following example. Suppose reader shield S2 and the substrate are separated by a thin insulator. Suppose S2 is at 1.5 V, and the substrate is at 6.5V. The difference is 5 V. If the space between them is 0.5 microns, the electric field (gradient) is 10V per micron, a very large value. For comparison, sparking in air, for example as observed on clothing, occurs from a gradient of approximately 1V per micron. In the example presented, the gradient is 10× that. Other unusual effects have been observed, including formation of solid water at room temperature in the presence of larger electric fields. Furthermore, the conductive grain structure of AlTiC concentrates the fields, which can be several times higher than in this example.
Several solutions have been contemplated, but each of these have drawbacks. These solutions include connecting the substrate directly to ground. However, if the reader shields are not clamped to ground, the voltage differential leads to the aforementioned tribological effects between the shields and substrate. Grounding the shields in a multi-sensor head is generally impractical. Even if the shields are grounded, adverse tribological effects may occur depending on the tape electrical and mechanical characteristics and other aspects of the tape path, such as ground or floating of guides.
Another problem encountered is that the readers are susceptible to shield-shorting which may occur in combination with substrate shorting, as a result of running magnetic recording tape having insufficient lubricity across the head at very low humidity, which in turn is found to produce accumulations of conductive material on the MR element, shields and substrate. Shorting is a well-known cause of reading errors. Proposed solutions, such as prerecessing and/or insulating heads, providing sensor-piggybacked fences and running ionization fans, require changes in head processing, design or implementation, respectively, and so are far more involved than the present invention. For instance, forcibly recessing the sensor so that its components do not develop the conductive accumulation is difficult to manufacture, and also generally produces undesirable spacing loss for the data readers, which must read much higher frequencies than the servo readers. Also, attempting to set shield potentials by tuning the MR bias could result in unacceptable bias values.
There is accordingly a clearly-felt need in the art for a magnetic device with reduced susceptibility to shorting and/or improved tribological characteristics. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.